1. Field of the Invention
The present invention relates to a magnetic recording medium adapted for use in a large-capacity memory, electronic filing systems and the like, as well as to a method and an apparatus for recording information on such medium.
2. Related Background Art
A magnetooptical recording medium is already known as an erasable recording medium. The magnetooptical recording medium is capable of recording with a small pit (of about 1 .mu.m) at a density about 100 times greater than when recording with in-surface magnetic recording media. Such media are expected to have applications as the files of high-quality images, recording moving images, etc., and the development of technology for further improving their recording density is strongly desired.
The above-mentioned magnetooptical recording medium is usually composed of a substrate and a magnetic recording layer formed thereon and has a magnetic anisotropy in a direction perpendicular to the surface of the substrate. Information recording is achieved by applying a magnetic field to the magnetic recording layer in a direction to invert the direction of magnetization and irradiating with a laser beam a pit area to be recorded to elevate the temperature of the pit area above the Curie temperature, Thereby, magnetization is inverted at the pit are. Also, information reproduction is achieved by irradiating the medium with a laser beam and detecting the information signal from the change in the state of polarization in the reflected or transmitted light. Such so-called Curie temperature recording, wherein information is recorded by inversion of magnetization by temperature increase of a vertical magnetization film beyond the Curie temperature in the presence of a biasing magnetic field, is considered to be suitable for high density recording.
However, for achieving a higher recording density, the binary recording of each bit by the direction of magnetization (upward and downward) of the magnetic recording layer becomes a limitation. The present recording density of the magnetooptical recording medium is largely determined by the wavelength of semiconductor laser (which is approximately equal to the beam diameter; about 800 nm at present). For example, to double the recording density of the media requires a semiconductor laser with a wavelength of 570 nm (800/.sqroot.2), but such a laser has not yet been developed. However, a doubled recording density is achievable if two magnetic recording films are superposed, whereby each bit has four values due to the combination of the directions of magnetization of the films. The recording/reproduction speed will also be doubled if the recording and reproduction can be conducted at the same speed as is used in conventional binary recording or reproduction. In fact, an even higher recording density or recording/reproduction speed is be achievable if it is possible to record a larger number of values.
There have already been proposals disclosed, for example, in Japanese Laid-Open Patent No. 59-116905 and U.S. Pat. No. 4,612,587, for multi-value recording by employing plural magnetic recording layers and controlling the direction of magnetization of each layer. However, such proposals have been associated with drawbacks such as requiring a laser source of multiple wavelengths, or excessively complicated recording medium construction or method recording/reproduction.